Understand the principle of air pressure braking and solve 80% of the problems of maintenance

Card friends know that the performance of the truck is closely related to its braking system, and the most problem with the motive transmission is that the vehicle cannot drive, but the failure of the brake system when driving often brings irreparable consequences.

For truckers, it is very important to have a certain understanding of how the braking system of the car works. This brief description of the principle of truck air circuits is hereby summarized, hoping to help card friends maintain and overhaul air circuits in the future. If there are improper introductions, please correct and discuss them, and learn and make progress together.

The working principle of the pneumatic assist brake system of common trucks is shown in the figure. The fresh air filtered by the air filter is first compressed and pressurized by the air compressor (air pump), and the gas output by the air compressor is a high-pressure gas with a certain temperature and humidity, which will cause damage to the pump valve when used directly.

Therefore, it must be cooled and filtered through condensing tubes and dryers. After that, the dry gas is distributed to each cylinder by the four-circuit protection valve, which is commonly known as the front axle brake cylinder, the rear axle brake cylinder, the trailer brake cylinder and the auxiliary gas cylinder.
 

The working principle of the air pressure assist brake system of common trucks

When the vehicle is braking, the driver presses the brake pedal to directly operate the brake valve (brake master cylinder), the brake valve inputs the air pressure signal to the relay valve of each bridge through the connecting pipeline, the control relay valve is connected to the brake air chamber (brake sub-pump) to supply air, the brake sub-pump is ventilated, and the brake sub-pump is ventilated and the mechanical structure such as the adjustment arm and camshaft rotates the shoes to rub the brake drum to achieve vehicle deceleration braking.

The air valves of the models produced by each manufacturer are different, but the main valves of the brake system are the same. The main pneumatic pump valves of common trucks are: four-circuit protection valve, foot brake valve, relay valve, quick-release valve, differential valve, load sensing valve, handbrake valve, etc.

▎1. Four-circuit protection valve

In order to ensure the reliability of the vehicle's braking system, multi-circuit joint braking is generally adopted. When a circuit pipeline fails to break and leak, the four-circuit protection valve can ensure that the air pressure in other circuits is not affected, and the brake system can still work without failure.

During work, the compressed air from the air compressor is supplied to each circuit reservoir after the four-circuit protection valve, and the whole vehicle gas circuit is divided into four circuits that are both interconnected and independent of each other.

Under normal circumstances, the four-circuit protection valve is actually a five-way connector, which only plays a protective role when a circuit is broken or leaked. The air outlet of the four-circuit protection valve is generally connected to the front axle brake cylinder, the rear axle brake cylinder, and the auxiliary gas cylinder.

▎2. Foot brake valve

The foot brake valve is the brake master cylinder that cardholders often say, which is the main control device in the pneumatic brake system, and the common brake valves are double-chamber brake valves with separate front and rear axle control.

When the vehicle brakes, the driver controls the brake pedal and the brake master cylinder is directly connected, the brake pedal stroke is used as the input signal, and the air pressure is controlled as the output signal to control the gas flow of the brake sub-pump and then determine the amount of braking force. An important feature of foot brake valves is the linear relationship between input and output.

At the time of design, some brake master cylinders have a small ratio of output and input in the front section and a large proportion in the rear section, so the driver will report that the effect of light brake pedaling is not obvious, but it is easy to lock up when pressed heavily. Of course, the characteristics of pneumatic braking are determined by the characteristics of the overall system, and it is also related to the subsequent relay valves and sub-pumps.

▎3. Relay valve and quick-release valve

In the air pressure brake system, the air reservoir, brake master cylinder and brake sub-cylinder are far apart, and the deflation and exhaust through the pipeline will lead to too long a lag time for braking and brake release, and the brake response is too slow.

The relay valve is usually installed on the air duct between the brake master pump and the brake sub pump and close to the brake sub pump, and the quick-release valve is installed after the relay valve. The relay valve can ensure the rapid inflation of the sub-pump when braking, and the quick-release valve can ensure the rapid discharge of the sub-pump when the brake is released.

▎4. Differential valve

The main function of the differential valve is to prevent the simultaneous use of the driving brake and the parking brake, and to avoid the overlap of the force of the brake air chamber and the spring gas chamber in the double-chamber composite brake sub-pump, so as to avoid the overload of mechanical components.

When the handbrake is applied, if the driver presses the driving brake at the same time, the differential valve will work, and the compressed air will enter the spring chamber of the brake sub-pump through the differential valve, release the parking brake in time to prevent the mechanical structure from overloading. In order to reduce the number of valves in the pneumatic system, differential valves and relay valves are usually designed as a combination valve, i.e. differential brake valve.

▎5. Load induction valve

The load-sensing valve is essentially a voltage regulator that can change the pressure of the rear axle brake circuit according to the load of the vehicle, installed on the frame, and the elastic arm of the load-sensing valve is connected to the axle, and the load of the vehicle is sensed by the bending degree of the axle steel plate.

If the brake pressure of the rear axle is the same in both cases, it will cause the rear axle to lock up or the full load braking weakness.

When the car is not loaded, the load sense valve can automatically reduce the braking force of the rear wheels, and when the car is fully loaded, the load sense valve can automatically increase the braking force of the rear wheels. To ensure that the braking force of the front and rear wheels meets the appropriate ratio when the vehicle is braking, and to ensure that the rear wheels do not lock up when the car brakes urgently.

▎Editor's remarks

Through the above introduction, card friends can realize that vehicle air braking is a relatively complex system, and the quality of braking performance is the overall effect of the combination of all valves and mechanical components in the system.

After understanding the working principle of the brake system, we must maintain the brake system reasonably and timely when using the vehicle, and frequent inspection and maintenance are important means to ensure driving safety and avoid unnecessary losses.